Apparatus for purifying metallic melts in a vacuum

ABSTRACT

Apparatus for purifying and degassing molten metals in vacuo, and continuously recycling the molten metal by means of barometric intake pipe and a barometric discharge pipe.

United States Patent De Bie et al.

[ 1 Sept. 5, 1972 [54] APPARATUS FOR PURIFYING METALLIC MELTS IN A VACUUM Inventors:

Assignee:

Filed:

App]. No.:

Edouard De Bie, Boris Tougarinoff, both of Antwerp, Belgium; Franz Sperner, Justus Moll, both of Hanan] Main, Germany Leybold-Heraeus Verwaltung GmbH, Cologne-Bayental, Germany and Metallurgie 'Hoboken, S.A., Belgium March 13, 1970 Foreign Application Priority Data March 14, 1969 Germany ..P 19 12 936.3

U.S. Cl.....

Int. Cl

.................... ..266/34 V, 75/49, 75/76} ..C2lc 7/10 Field of Search ..13/31; 75/49, 76; 266/34 V Apparatus for purifying and degassing molten metals Primary Examiner-Gerald A. Dost Att0rney.loseph F. Padlon ABSTRACT in vacuo, and continuously recycling the molten metal by means 'of barometric intake pipe and a barometric discharge pipe.

6 Claims, 4 Drawing Figures PAIENIEBsEP 5 m2 SHEET 2 BF 4 APPARATUS FOR PURIFYING METALLIC MELTS IN A VACUUM The invention relates to a method and apparatus for purifying metallic melts and of degassing them in a vacuum. It applies particularly to melts of copper, wherein the melt is fed continuously from a container for the molten bath by means of a barometric intake pipe to a device for vacuum treatment located above the melt, and which thereafter flows back to the same molten bath container through a barometric discharge pipe. Copper must be thoroughly degassed, since traces of oxygen would decrease electrical conductivity. In other words, copper is a very critical material which is a subject of the invention.

Such a method, also described as degassing by circulation, is known, for example, from the published German patent application No. 1,216,904. According to said patent, however, degassing is performed in a single vacuum chamber which is disadvantageous particularly because an appreciable amount of conveying gases must be added to the metallic melt in the barometric intake in order to produce the desired flow. The conveying gas is liberated in the vacuum chamber and causes a significant deterioration of the vacuum. No significant improvement is achieved by extending the device degassing time by circulation relative to the amount of processed metal, because firstly, there is a cooling of the melt, and secondly, the degassing equilibria are still quickly established. Such a degassing process is not applicable to a melt continuously supplied with and delivered of large amounts of metal, or it is applicable only with a very inadequate result.

The known methods are all based on a relatively rapid passage of the melt through the single degassing chamber. This causes the disadvantage of poor or no degassing of the lower layers of the metal flowing from the barometric intake pipe to the discharge pipe because of inadequate mixing and of the small surface area of the melt which it is desired to degass. It is an additional disadvantage of the known methods that there are no provisions for purifying the melts of foreign bodies and slags which are present in the solid or liquid states. Failure to comply with the last mentioned requirement may not have overly serious consequences in steel melts for which the known methods are primarily intended, but causes an unacceptable deterioration of the material in the production of high-purity copper.

A two-stage method for degassing metals disclosed in US. Pat. No. 3,367,396 causes a portion of the metal to be led in a cyclic path, the pressure decreasing in the direction of flow, but the second stage employs the socalled pouring stream degassing method. The latter method requires a certain drop, and therefore a spatial separation of the two degassing stages. This makes it impossible to build a compact apparatus so that heat losses through the large surface are substantial even when good thermal insulation is employed, and are not tolerable for an extended use of the degassing process. Moreover, the pouring stream degassing method has the disadvantage of a short dwell time so that the complete removal of all gases from the melt is not always possible.

The object of the invention is the avoidance of the disadvantages resulting from the known methods, and particularly the provision of a method for copper purification which results in a totally degassed ultimate product of high purity.

This object is achieved by the invention by performing the degassing operation in at least two degassing chambers adapted to be separately evacuated and located on the same level, the two chambers being passed sequentially by the stream of molten metal and sealed from each other by the metal against transfer of gas therebetween. This arrangement has the advantage that a much higher vacuum can be reached in the second chamber because of the absence of conveying gas, and that the molten metal can be purified simultaneously in the second chamber of particles of foreign matter and of slag in the solid or liquid state. The power requirements and dimensions of the pumping units are held within tolerable limits. It is possible not only to reach a higher vacuum in the second chamber, but also, depending on the capacity of the chamber to achieve an adequate dwell time for the melt. The arrangement of the chambers on a common level results in the particular advantage of a compact structure because both chambers can be enclosed in a common insulating shell. Also, the heat losses are greatly reduced thereby.

A preferred apparatus for performing the method is arranged in such a manner that the device for vacuum' treatment consists of two degassing chambers separated by a partition wall, a liquid seal being interposed between the chambers and filled with the molten metal, each of the chambers communicating with a barometric pipe which is immersed in the melt to be degassed. In order to increase the exposed metal surface by extending the flow path, an inclined, step or wave shaped flow area for the molten metal is preferably provided with the second degassing chamber.

Furthermore, according to the invention, the

degassing chambers as well as the barometric pipes may be arranged substantially concentrically, the liquid seal being annular. The first, inner degassing chamber is enveloped by the second, outer chamber. Because of the greater radius of the second chamber, it s possible to provide there not only a longer flow path for the molten metal, but also a larger volume so that an optimal dwell time for the melt can be adjusted. Moreover, there is achieved a particularly simple design of the apparatus with a minimum of thermal losses. Because the basic elements have rotational symmetry, there is obtained an apparatus of increased mechanical strength. The inner barometric pipe is preferably employed as the suction pipe, and its upper end is provided with a dish shaped enlargement having an annular groove offset from its outer circumference, the groove jointly with the partition of hollow cylindrical shape bounding the space for the liquid seal. The dish shaped enlargement constitutes a so-called downflow skirt for the melt, the surface of the melt being enlarged particularly at the outer rim of the skirt, and the release of gas from the melt being greatly facilitated by the formation of a film having two exposed surfaces as the melt runs off the rim.

A preferred embodiment of apparatus for performing the method according to the invention are described hereinafter with reference to FIGS. 1 to 4 which show:

FIG. 1 l is a sectional view of the embodiment of the invention showing a melt container with different superimposed degassing devices;

FIG. 2 is a sectional view of a modification of the embodiment shown in FIG. 1;

FIG. 3 is a section of a device with several sequentially arranged degassing devices according to FIG. 1 or FIG. 2; and

FIG. 4 is a schematic representation of a device with several degassing devices according to FIG. 1 or FIG. 2, arranged in parallel.

In the drawings, specific reference numerals refer to like parts thereof.

' FIG. 1 shows a melt container 1 which is a warming furnace to keep the melt warm. It is equipped with an induction heater 2 for the molten metal 3. A degassing apparatus generally indicated by numeral 4 is superimposed on the warming furnace. Said degassing apparatus has two degassing chambers 5,6 juxtaposed at the same level and separated from each other by an upright partition wall 7. The bottom of degassing apparatus 4 has a groove shaped recess 8 partly engaged by a partition 7 extending thereinto. The cooperation of these two structural features will be described in more detail below. The upper ends of the degassing chambers 5,6 are connected to vacuum lines 9,10, which lead to separate pumps or sets of pumps, not shown.

A barometric suction conductor pipe 11 terminates in the bottom portion of the degassing chamber 5, while the bottom portion of degassing chamber 6 leads to the barometric discharge pipe 12. Moreover, a supply line 13 for injecting a conveying gas inert to metal, such as nitrogen, into the metal melt, leads into the barometric feed pipe 11. An additional settling space 14 is provided between degassing chamber 6 and barometric discharge pipe 12.

In said chamber 6, there is provided an inclined, stepped flow surface 15 whose lower end terminates in the settling space 14 and communicates with the groove shaped recess 8 in said second degassing chamber 6. The lower ends of barometric pipes 11,12 dip into the molten metal 3 and thereby seal the degassing chambers 5,6 from the atmosphere.

In order to prevent entry of the melt or of splashed metal into the vacuum lines 9,10, liquid separators 17,18 arranged in front of or below the orifices of the lines have the shapes of plates about which gas may freely flow. The melt container as well as the degassing apparatus 4 are provided with a lining 19 which consists of graphite or a graphite bearing material from which partition 7, pipes 1 1,12 and separators 17,18 are also made. With the exception of the barometric pipes, all parts of the apparatus are enveloped by a metallic shell 20. A non-illustrated layer of thermally insulating material is preferably interposed between the shell and the liner 19.

The apparatus according to the invention operates as follows:

When a vacuum is applied to degassing chambers 5,6, the liquid metal first rises in the intake and discharge tubes 11,12 because of the barometric pressure differential without any liquid transfer from chamber 5 to chamber 6 even when the chamers are only partly filled. Because of the liquid contained in the chambers, partition 7 jointly with the groove shaped recess 8 forms a so-called liquid seal 16 which prevents interchange of gases between the two degassing chambers 5,6. This is necessary because both chambers are preferably adjusted to different pressures, the vacuum in the chamber 6 being higher than in the chamber 5.

When now a conveying gas inert to the metal melt, such as nitrogen or helium, is blown into the melt in the barometric intake pipe 11 through the supply line 13, the liquid level in the degassing chamber 5 rises by a certain amount because of the lower specific gravity of the gas bearing melts, the contents of the chamber being simultaneously agitated vigorously and mixed thereby. The entire conveying gas together with a portion of the original gas content of the melt is withdrawn from the degassing chamber 5 through the vacuum line 9. Because of the level difference in the two degassing chambers, molten metal is transferred through the liquid seal 16 whereby the melt is again agitated and well mixed. This mixing action continues on the inclined, stepped flow surface 15 where a practically complete degassing is achieved because of the much higher prevailing vacuum. The thin melt layer which forms on the surface 15 greatly favors the release of gas, and particles of foreign matter and slag simultaneously are brought to the surface of the melt. Behind the flow surface 15, the melt enters the settling space 14 and is returned from there to the melt container 1 by the barometric discharge pipe 12. During settling of the melt in the settling space 14, the entrained particles of foreign matter and slag rise to the surface of the melt where they accumulate and are removed from time to time.

In FIG. 2, there is shown a modified form of the device of FIG. 1 in which elements having the same function are designated by the same numerals, with suffix 0 added. The barometric intake and discharge pipes 11a, 12a as well as the degassing chambers 5a,6a are concentric cylindrical elements in this case. The central barometric intake pipe 11a is provided with a dish shaped enlargement 22 at its upper end, the enlargement being provided with an annular recess 23 spaced from the rim thereof. Recess 23, jointly with the lower end of the cylindrically tubular partition wall 7a, provides the space for the liquid seal 16a. The lower end of the cylindrical partition wall 7a is provided with arcuate recesses which constitute well-defined passages for the molten metal. The lands of the partition 7a between the recesses rest on the dish shaped enlargement 22 in the recess 23. The liquid separators 17a which are perforated plates are located within the partition and pass the entire conveying gas and a portion of the gas originally present in the metal to the vacuum line 90, and thence to the pumps.

After passing the liquid seal 16a, the melt runs in a thin film over the flow surface 15a of the dish shaped enlargement 22, and from there into the settling space as a freely falling film 24 exposed on both sides to the vacuum in degassing chamber 6a. Degassing chamber 6a communicates with vacuum line 10a and the associated pumping plant through the liquid separator 18a which is also a perforated plate. The melt returns from the settling space 14a through an annular space bounded between the barometric discharge pipe 12a and the barometric intake pipe 110. Barometric discharge pipe 12a is provided at its lower end with a bottom 25 in which the lower end of the intake pipe 11a is inserted in a liquidtight engagement. The metal melt therefore must leave the barometric discharge pipe 12a through radial discharge openings 26. Said discharge openings 26 are not located too closely adjacent the intake opening of the barometric intake pipe 11a, but it is immaterial whether a portion of the circulating melt is'again drawn into the intake pipe immediately after leaving the discharge pipe because a high rate of circulation of the melt is beneficial in causing the entire contents of the melt container la to participate in a short period in the purification process in the degassing device 4a. Heat losses from degassing device 4a are reduced by special heat insulation 21 which may consist of glass wool, rock wool or graphite wool, and which in turn is surrounded by a metallic shell 20. The degassing apparatus and the operating personnel are additionally protected by a radiation protection device 27 comprising several sheet metal shields.

It is to be noted that the mode of operation of the apparatus shown in FIG. 2 is the same, in principle, as that of the device of FIG. 1.

The method and apparatus of the invention are eminently suitable for intermittent operation, that is, for the degassing of a batch located in a melt container, as well as for continuous operation. When operated continuously, the degassing apparatus communicates with a continuously operating melting plant and also with a continuously operating apparatus for the casting of continuous billets or ingots. A continuously operating installation is diagramatically illustrated in FIG. 3. Three melt containers la, 1b, I I c, are provided with respective superposed degassing devices 40, 4b, 4c and are arranged in series. The first melt container communicates with the continuously operating melting apparatus 29 by way of a trough 28. The last melt container supplies a continuously operating billet or ingot casting plant 31 through control valves 30. The molten metal 3 flows from a preceding melt container over an overflow 32 to the subsequent melt container. The arrangement of FIG. 3 is employed if the ultimate product is required to be a metal of extreme purity and freedom from gases. It is possible to closely approach the theoretical limit of purity if an oxidizing gas such as air, oxygen and/or chlorine or chlorine compounds are fed to the first stage in the melt container 1a and/or in the degassing device 40 together with the conveying gas. The oxidizing gases facilitate the removal of such impurities as lead, arsenic, antimony, selenium, tellurium and sulfur down to an amount of 1 part per million or less. The second stage (1b to 4b) is supplied with a reducing gas, such as hydrogen, carbon monoxide, a hydrocarbon and/or chlorine or a chlorine compound which permits a metal to be obtained completely free from oxygen. In the third device (lc to 4c) according to FIG. 3, the metal is mixed with a neutral gas, such as nitrogen, to remove all residual gases present other than oxygen. Casting may then follow.

If the requirements for purity of the treated metal are less stringent, it is possible, of course, to replace the three, series arranged degassing devices 4a,4b,4c by a single degassing device and a single melt container which is passed by the continuously supplied molten metal.

FIG. 4 diagrammatically illustrates a parallel arrangement of three melt containers 1a,lb,1c with superposed degassing devices 4a,4b,4c.- This arrangement permits a fully continuous operation, degassing being performed in one of the devices while casting and/or supplying the metal to be purified takes place in the other one. The arrangement of FIG. 4 further permits subjecting the metal in the several melt containers and/or the associated degassing devices to various chemical treatments or reactions, the several portions of the melt so treated being led together and mixed prior to pouring into molds. It is possible, for example, to modify the treatment by introducing first an oxidizing agent and subsequently a reducing agent into the one device or to proceed in the opposite sequence as described above. This permits obtaining an ultimate product of desired specific properties.

The examples illustrated in FIGS. 3 and 4 do not exhaust the possible combinations of individual degassing devices. It is entirely possible in certain cases only two melt containers with superposed degassing devices may be required for achieving a pure metal or specific properties. On the other hand, combinations of four or more sets, and combinations of series-arranged and parallel sets are also possible. Obviously, the method of the invention is applicable not only to a continuous casting process, but also to the casting of individual shapes.

What is claimed is:

1. Apparatus for purifying and degassing molten metals by vacuum comprising, in combination, a melt container; a device located above the melt for vacuum treating said melt; means for continuously feeding said melt from said container to said device by a conveying gas; barometric intake pipe means communicating with said melt container for passing said melt from said container to said device; barometric discharge pipe means communicating with said melt container for returning said melt to said container after vacuum treatment by said device; and two degassing chambers in said vacuum device located on a common level and being separately evacutable, said two chambers being passed in sequence by the flowing melt and being sealed gastight from each other by said melt, said melt forming an annular liquid seal between said chambers, said degassing chambers being substantially concentric with said barometric pipe means.

2. Apparatus according to claim 1 wherein an inclined, stepped wave shaped ilow surface for the molten metal is provided in the second degassing chamber behind said liquid seal.

3. Apparatus according to claim 1 wherein the inner barometric pipe serves as suction pipe and is provided at its end with a dish shaped enlargement having an annular recess spaced from the rim of the enlargement, the recess jointly with the lower end of the cylindrically tubular partition bounding the space for the liquid seal.

4. Apparatus according to claim 1, characterized in that several of the devices for vacuum degassing are arranged above melt containers which are arranged in series as for continuous cascading flow therethrough.

5. Apparatus according to claim 1, characterized .in that several of the devices for vacuum degassingare arranged above melt containers arranged in parallel for continuous flow therethrough.

pipe means in the bottom of the degassing chambers to permit the flow of the melt from one chamber to the other and supply means for one of said chambers for injecting a gas inert to the metal to recycle the melt from the chambers into the melt container.

* l l l IF 

1. Apparatus for purifying and degassing molten metals by vacuum comprising, in combination, a melt container; a device located above the melt for vacuum treating said melt; means for continuously feeding said melt from said container to said device by a conveying gas; barometric intake pipe means communicating with said melt container for passing said melt from said container to said device; barometric discharge pipe means communicating with said melt container for returning said melt to said container after vacuum treatment by said device; and two degassing chambers in said vacuum devIce located on a common level and being separately evacutable, said two chambers being passed in sequence by the flowing melt and being sealed gas-tight from each other by said melt, said melt forming an annular liquid seal between said chambers, said degassing chambers being substantially concentric with said barometric pipe means.
 2. Apparatus according to claim 1 wherein an inclined, stepped wave shaped flow surface for the molten metal is provided in the second degassing chamber behind said liquid seal.
 3. Apparatus according to claim 1 wherein the inner barometric pipe serves as suction pipe and is provided at its end with a dish shaped enlargement having an annular recess spaced from the rim of the enlargement, the recess jointly with the lower end of the cylindrically tubular partition bounding the space for the liquid seal.
 4. Apparatus according to claim 1, characterized in that several of the devices for vacuum degassing are arranged above melt containers which are arranged in series as for continuous cascading flow therethrough.
 5. Apparatus according to claim 1, characterized in that several of the devices for vacuum degassing are arranged above melt containers arranged in parallel for continuous flow therethrough.
 6. Apparatus for purifying and degassing metals comprising a melt container, heating means for melt, degassing means connected to said container, said degassing means having at least two degassing chambers juxtaposed at the same level, partition means between said chambers, said degassing means having a recess for said partition means and barometric suction pipe means in the bottom of the degassing chambers to permit the flow of the melt from one chamber to the other and supply means for one of said chambers for injecting a gas inert to the metal to recycle the melt from the chambers into the melt container. 